A spark plug voltage detector for use in internal combustion engines

Abstract: A spark plug voltage waveform detector for use in internal combustion engine, having an electrode plate (21) embedded in an insulator body (2) to be electrically connected to a lead wire (22). A pair of insulator plates (31,32) are located adjacent the insulator body (2), and each of the insulator plates has a groove in which is placed a spark plug cable (H C ) of a secondary circuit to form static capacity between the cable (H C ) and the electrode plate (21) so as to detect a spark plug voltage waveform. Electromagnetic shield plates (4,5) are provided to liquid-tightly sandwich the insulator body (2) and the insulator plates (31,32) by means of bolt and nut (6). The electromagnetic shield plates (4,5) are electrically connected to an internal combustion engine for grounding.

Description: This invention relates to a spark plug voltage detector which is provided to detect a spark plug voltage waveform induced from a secondary circuit of an ignition circuit and to be applied to a spark plug in an internal combustion engine. With the demand of purifying emission gas and enhancing fuel efficiency of internal combustion engines, it is desirable to detect firing conditions in each cylinder of an internal combustion engine so as to protect the internal combustion engine against any type of misfire. In order to detect the firing condition in each of the cylinders, the applicant of the present patent application discloses a misfire detector device in Japanese Patent Appln. No. 3-314783 see also EP-A-0 544 548. This device is based on the understanding that a misfire in a cylinder may be detected from the voltage waveform across the spark plug, and by analysing its attenuation characteristics. On this basis, an electrode plate sensor is embedded in an insulator body which has a plurality of grooves which have the same diameter as a spark plug cable . The spark plug cable is placed in the grooves to form static capitance between the electrode plate sensor and the cable. A lid plate is secured to the insulator body to fix the cable by means of a screw. In another Japanese Patent Appln. No. 4-303859, an electrode plate is placed in an accommodation space filled with silicone rubber. A spark plug cable is sandwiched by a pair of rubber plates, and placed in grooves which are diametrically equivalent to the spark plug cable. These rubber plates interfit with a cavity provided with the accommodation space. In these types of sparkplug voltage detector probes, it is quinteessential to detect the change of the static capitance formed between the electrode plate and the spark plug cable. Upon inadvertently permitting entrance of foreign matter such as oil, water, dust, soil and the like between the electrode plate and the spark plug cable, it is difficult to precisely detect the true static capacity therebetween. In order to prevent the entry of the foreign matter , the diameter of each groove is chosen to be the same as the spark plug cable, and the spark plug cable is secured in the groove by means of adhesive. Otherwise, the rubber plate is used to ensure a certain tightness between the grooves and the spark plug cable by their elasticity. In the case where the rubber plates tightly sandwich the spark cable due to their elasticity, it is found that the tightness between the grooves and the spark plug cable becomes insufficient particularly when sandwiching a plurality of spark plug cables extended from the internal combustion engine. In the case where the spark cable is placed in the grooves of the insulator body by means of adhesive, it is necessary to disadvantageously replace the detector probe itself upon exchanging the spark plug cable. Therefore, it is an object of the invention to provide a spark plug voltage detector probe for use with an internal combustion engine which allows easy separate replacement of components, and which precisely detects characteristics of the spark plug voltage waveform applied to each spark plug mounted on the respective cylinders with a relatively simple structure. According to the invention, there is provided a spark plug voltage waveform detector for use with a secondary circuit of an ignition circuit of an internal combustion engine to form a static capacitance therebetween and to detect a spark plug voltage waveform, the detector comprising: an electrode plate embedded in an insulator body to be electrically connected to a lead wire; a pair of insulator plates to be located adjacent the insulator body, the opposing surfaces of the insulator plates respectively having at least one groove for receiving a spark plug cable of the secondary circuit so as to sandwich the spark plug cable therebetween; and a pair of electromagnetic shield plates provided to sandwich the insulator body and the pair of insulator plates. The electromagnetic shield plates are preferably made of aluminum or an alloy thereof or another appropriate conductor. With the spark plug cable sandwiched by the insulator plates, it is possible to tightly fix the cables extended from the internal combustion engine in position. With the use of the electrical shield plates, the bolt and nut, it is possible to maintain liquid tightness sufficiently to prevent the entry of the foreign matter between the spark plug cable and the electrode plate, and at the same time, prevent entry of external disturbance noise which would otherwise affect the static capacity between the spark plug cable and the electrode plate. This makes it possible to precisely detect the spark plug voltage waveform applied to the spark plug with a relatively simple structure. With the spark plug cable and the electrode plate of the insulator body detachably laminated by the cable fixing means, it is economically advantageous to replace only the cable fixer portion without exchanging the insulator body upon replacing the spark plug cables which deteriorates more readily than the electrode plate of the insulator body. The present invention will be more clearly understood from the following description, given by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a transverse cross sectional view of a spark plug voltage waveform detector probe according to an embodiment of the invention; Fig. 2 is an exploded perspective view of the spark plug voltage waveform detector probe; Fig. 3 is a perspective view of the spark plug voltage waveform detector probe mounted on an internal combustion engine; Fig. 4 is an exploded perspective view of the spark plug voltage waveform detector probe into which an electronic circuit is incorporated; and Fig. 5 is a connecting diagram of an ignition circuit into which the spark plug voltage waveform detector probe is incorporated. Referring to Figs. 1, 2 which show a spark plug voltage waveform detector probe 1 according to an embodiment of the invention, an electrical insulator body 2 is provided to have an electrode plate 21 embedded therein placed round the insulator body 2, are a cable fixer portion 3, electromagnetic shield plates 4, 5 and a cable fixing means 6 which tightly sandwich the insulator body 2 and cable fixer portion 3 by way of the electromagnetic shield plates 4, 5. From the electrode plate 21 embedded in the insulator body 2, a lead wire 22 is introduced outside so as to form a connector 23 with an open box 23a integrally made in a manner to surround the lead wire 22. In this instance, the electrode plate 21 is embedded in the insulator body 2 simultaneously when the insulator body 2 is made by means of injection moulding. The shield plate 4 is tightly secured to an upper surface of the insulator body 2. In the meantime, the cable fixer portion 3 has a pair of electrical insulator plates 31, 32 by which a spark plug cable (Hc) is sandwiched. An outer marginal part of the insulator plate 31 has a frame 31a which surrounds an outer periphery of the insulator plate 32 so as to readily place the insulator plate 31 in position when laminated each other. Opposing surfaces of insulator plates 31, 32 have an array of grooves 33, 34 in which the spark plug cable (Hc) is tightly placed. In this instance, the spark plug cable (Hc) may be provisionally placed in the grooves 33, 34 through an adhesive or the like. It is further predetermined that the diameter of grooves 33, 34 are substantially the same as that of the spark plug cable (Hc). The electromagnetic shield plates 4, 5 are made of an aluminum flat metal so as to absorb electromagnetic wave induced by a high voltage applied to the spark plug, in particular by an intensified current flow caused from a capacitive discharge. The absorption of the electromagnetic wave makes it possible to neutralize an influence of noise on the static capacitance between the electrode plate 21 and the spark plug cable (Hc). From the shield plate 4, an anchor tool 41 integrally extends, this being formed at the time of moulding the shield plate 4. Through the anchor tool 41, the shield plate 4 is secured to an internal combustion engine (E), and at the same time, adapted to be grounded to the earth. In this instance, it is observed that the anchor tool 41 may be provided on the shield plate 5 from the reason that the shield plates 4, 5 are electrically connected by means of the cable fixing means 6. The shield plates 4, 5 sandwich the insulator body 2 and the cable fixer portion 3 which are tightly laminated to each other physically by means of the cable fixing means 6. The cable fixing means 6 includes a metallic nut 62 and bolt 61 which secure one shield plate 4 to another shield plate 5 so as to maintain liquid-tightness between the insulator body 2 and the cable fixer portion 3. The cable fixing means 6 serves as not only a securement but also a conductor which electrically connect between the shield plates 4, 5 to absorb the electromagnetic wave so as to protect the detector probe 1 against the disturbance noise. In the instance where the insulator body 2 and the cable fixer portion 3 are tightly laminated by means of the cable fixing means 6, it is necessary to make at least one of the insulator body 2 and the insulator plate 31 from an elastic material such as rubber, synthetic resin or the like in order to maintain the liquid-tightness of an interface between the insulator body 2 and the cable fixer portion 3. The spark plug voltage waveform detector probe 1 is assembled as shown in Fig. 2, and mounted on the internal combustion engine (E) by way of the anchor tool 41 as shown in Fig. 3. The spark plug cable (Hc) electrically connects a spark plug (P) to a distributor (D) by way of the grooves 33, 34 of the cable fixer portion 3 of the spark plug voltage waveform detector probe 1. Fig. 4 shows an electronic circuit 7 of a spark plug voltage detector device incorporated into the spark plug voltage waveform detector probe 1 according to a modified form of the embodiment of the invention. In the modified form of the embodiment of the invention, an electromagnetic shield case 71 is made by aluminum die cast (rectangular parallelepiped), and provided between the shield plate 4 and the cable fixer portion 3. An upper space of the electromagnetic shield case 71 serves as an accommodation room 73 for a circuit board 72, and a lower space of the shield case 71 serving as a rectangular cavity 74 which accommodates the electrode plate 21 embedded in the insulator body 2 by means of an adhesive or the like. On a bottom wall of the shield case 71, a throughhole 75 is provided to introduce a lead wire 70. In this instance, the insulator body 2 and the cable fixer portion 3 are protected by the shield plate 5 and the bottom wall of the shield case 71. This enables to obviate the necessity of integrally moulding the shield plate 4 at the time of making the insulator body 2 by means of injection moulding. From the reason that the shield plate 4 provided to serve as a lid and shield for the accommodation room 73 and the electronic circuit 7 respectively. This enables to neutralize not only the influence of the noise on the static capacitance between the electrode plate 21 and the spark plug cable (Hc) but also on the electronic circuit 7 of the spark plug voltage detector device. Fig. 5 shows an ignition circuit 100 into which the spark plug voltage detector probe 1 is incoporated. The ignition circuit 100 has ignition coil T which includes a primary circuit L1 and secondary circuit L2 with a vehicular battery cell (V) as a power source. The primary circuit L1 have primary coil 1a electrically connected in series with a signal generator (SG), while the secondary circuit L2 have secondary coil 1b electrically connected to the distributor (D) through which a high voltage is applied to the spark plug (P) mounted in each cylinder of the internal combustion engine (E). Between the lead wire of the electrode plate of the spark plug voltage detector probe 1 and the ground, is a voltage divider circuit 8 connected which consists of a capacitor C (3000 pF) and a resistor R (3 MΩ) each connected in parallel. The voltage divider circuit 8 is connected to a spark plug voltage waveform detector circuit 9 which is connected to a distinction circuit 10 of a microcomputer. When the ignition coil (T) supplies the high voltage with the spark plug (P) by way of the distributor (D) so as to provide the spark plug voltage, the static capacity formed between the electrode plate and the spark plug cable reaches to 1 pF. The voltage divider circuit 8 divides the spark plug voltage induced from the secondary circuit L2 by the order of 1/3000, which makes it possible to determine the time constant of RC path to be approximately 9 milliseconds to render an attenuation time length of the spark plug voltage waveform relatively longer (approx. 3 milliseconds). The sparkplug voltage waveform detector circuit 9 detects a sparkplug voltage waveform on the base of the output from the voltage divider circuit 8. The divided voltage waveform by the voltage divider circuit 8 is directly analogous to the spark plug voltage waveform applied to the spark plug (P). In the course of operating the internal combustion engine (E), it is found that the voltage waveform changes depending on whether or not the established spark discharge fires the air-fuel mixture gas, whether or not a misfire occurs in the cylinder of the internal combustion engine (E) and whether or not the jump spark miss occurs due to the deteriorated ignition coil (T), cabon fouling of the spark plug (P) or excess discharge of the vehicular battery cell (V). After the spark plug voltage waveform detector circuit 9 analyze the voltage waveform, the distinction circuit 10 compares the voltage waveform with data previously determined by theoretical calculation or experimental test result, and distinguishing the normal combustion from the misfire and the jump spark miss. Depending on these conditions, the distinction circuit 10 generates different amount of output to feed it to a main computer which control the internal combustion engine (E), or directing feeding a control signal to a fuel injector and an ignition timing control device (not shown). It is noted in the embodiment of the invention, the capacitor (C) and the resistor (R) of the voltage divider circuit 8 are 3000 pF and 3 MΩ respectively, however, the capacitor (C) and the resistor (R) are not limited to 3000 pF and 3 MΩ, but may be altered at discretion as required. It is as a matter of course that the cross section of the spark plug cable (Hc) may be not only circular but also triangular, rectangular and polygonal, the cross section of the groove corresponding to that of the spark plug cable. It is also appreciated that the insulator body 2 and the cable fixer portion 3 may be made of silicone rubber or other type of plastic materials respectively. It is appreciated that the cable fixing means 6 may be made from an aluminum metal. It is further observed that an entire surface of the electromagnetic shield case 71 may coated by means of aluminum deposit or evaporation instead of making it of the aluminum metal. It is appreciated that instead of using the nut and bolt combination, the shield plate 4 may directly be secured to the shield plate 5 by means of a screw stud. It also stands as a matter of course that each entire surface of the shield plates 4, 5 may be coated by means of aluminum deposit or evaporation instead of making them by the aluminum metal. While the invention has been described with reference to the specific embodiments, it is understood that this description is not to be construed in a limiting sense in as much as various modifications and additions to the specific embodiments may be made by skilled artisan without departing from the scope of the invention.

Claims: A spark plug voltage waveform detector for use with a secondary circuit of an ignition circuit of an internal combustion engine to form a static capacitance therebetween and to detect a spark plug voltage waveform, the detector comprising: an electrode plate (21) embedded in an insulator body (2) to be electrically connected to a lead wire (22) ; a pair of insulator plates (31,32) to be located adjacent the insulator body (2), the opposing surfaces of the insulator plates (31,32) respectively having at least one groove (33,34) for receiving a spark plug cable (H C ) of the secondary circuit so as to sandwich the spark plug cable therebetween; and a pair of electromagnetic shield plates (4,5) provided to sandwich the insulator body (2) and the pair of insulator plates (31,32). A detector according to claim 1 further comprising fixing means (6) for holding the pair of electromagnetic shield plates (4,5) in the sandwich arrangement. A detector according to claim 2 wherein the fixing means (6) electrically connects the pair of electromagnetic shield plates. A detector according to any preceding claim arranged to liquid tightly sandwich together the insulator body (2) and the pair of insulator plates (31,32). A detector according to any preceding claim wherein the electromagnetic shield plates are made of aluminum or an alloy thereof. A detector according to any preceding claim wherein the electromagnetic shield plates are arranged to be electrically connected to an internal combustion engine for grounding. A detector according to any preceding claim further comprising an electromagnetic shield case (71) to be sandwiched between one of the electromagnetic shield plates (5) and the insulator body (2), the case (71) containing an electronic circuit (7) of a spark plug voltage detector device. A detector according to any preceding claim further comprising an extension (41) for mounting the detector to an internal combustion engine.

Inventors: KONDO NORIAKI, MIYATA SHIGERU, KONDO, NORIAKI, MIYATA, SHIGERU, KONDO, NORIAKI, C/O NGK SPARK PLUG CO.,LTD., MIYATA, SHIGERU, C/O NGK SPARK PLUG CO.,LTD.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1994
Main classification: F02P17
IPC codes: F02P17, G01R31